In the field of orthodontics there has been an increasing demand for brackets that are aesthetic. In response to this need, various materials have been suggested for use in orthodontics brackets. Plastic materials have been found to be unsatisfactory due to their inability to provide the appropriate strength necessary for an orthodontic bracket. Various ceramic materials have been suggested in the prior art. However, prior art brackets made from these materials are quite brittle and are subject to fracture. Orthodontic brackets made of single crystalline alumina or cubic zirconia have been suggested. While these have been found to provide the strength necessary for use as an orthodontic bracket, they are relatively expensive materials, and are relatively difficult to machine due to their hardness and are subject to fracture due to their extreme hardness.
It has also been suggested to make orthodontic brackets from an ion exchange strengthened glass, such as illustrated and set forth in U.S. Pat. No. 4,784,606. This patent discloses strengthening the glass by subjecting it to an ion exchange reaction. The ion exchange treatment is carried out by immersing the glass bracket in a bath of molten salt at elevated temperatures above the strain point and below the softening point of the glass. The treatment time can vary from 2-24 hours. After it is removed from the bath, the bracket is washed clean of excessive salt. This, of course, adds substantial manufacturing cost. Additionally, the ion layer formed is very thin, and as a result, is subject to early wear which can result in failure of the part due to stress risers that can form.
It has been further suggested in the prior art to produce orthodontic brackets of a glass ceramic having dual crystalline structure, such as illustrated in U.S. Pat. No. 4,789,649. This patent discloses a bracket structure having relatively large crystals covered by a layer of smaller, flat crystals. While this structure may provide increased strength, its central crystalline structure is relatively difficult to machine and is subject to fracture in the same manner as other crystalline type brackets of the prior art.
Applicants have invented an orthodontic bracket which is made of a relatively inexpensive glass which can provide the desired strength, is easy to manufacture, relatively resistant to fracture, and does not require bath solutions or cleaning operations in order to enhance the strength of the material.